Refractory metal base alloys and method of making same



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22, 1964 c. D. DICKINSON ETAL 3,161,949

REFRACTORY METAL BASE ALLOYS AND METHOD OF MAKING SAME (so f PERCENT TITANIUM INVENTORS CLAYTON D. DICKINSON ROBERT STEINITZ ATTORNEY United States Patent 3,161,949 REFRACTORY MET BASE ALLOYS AND METHOD OF MAKING SAME Clayton 1). Dickinson, Port Washington, and Robert Steinitz, Harrison, N.Y., assignors to General Telephone and Electronics Laboratories, Inc., a'corporation of Delaware Filed Feb.. 21, 1963, Ser. No. 260,963 7 Claims. (Cl. 29182.5)

This invention relates to refractory metal base alloys and in particular to alloys of molybdenum and tungsten with a reactive element.

It' is an object of our invention to produce molybdenum and tungsten base alloys having high temperature properties that are superior to those of known alloys of this type. Specifically, the alloys we have produced exhibit higher recrystallization temperatures and greater strength at elevated temperatures than known refractory metal base alloys. Q

In preparing conventional molybdenum and tungsten base alloys, considerable care is normally exercised to minimize the amount of nitrogen in the alloy. For example, proposed specifications of the American Society of Testing Materials state that the maximum permissible amount of nitrogen in molybdenum base alloys containing 0.5 percent titanium is 10 parts per million. These specifications include :molybdenum and molybdenum alloy bar, rod and wire, as well as other forms of molybdenum and molybdenum alloys. However, we have found surprisingly, that by dispersing relatively large quantities of nitrogen through a refractory metal base alloy, the recrystallization temperature is substantially increased and that the tensile strength is also higher than that of alloys from which nitrogen has been excluded.

The amount of nitrogen in our alloys is between 200 parts per million and the amount required to transform all the reactive metal into a compound with nitrogen. For example, in a molybdenum base alloy containing 0.5 percent titanium, thenitrogen content is between 200 and1460 parts per million, a nitrogen content which is considerably higher than that found in previously known molybdenum-titanium alloys.

In producing our alloys, the elemental powder of molybdenum or tungten is mixed with that of a reactive element, such as titanium, hafnium, or zirconium, selected from Group IVA of the Periodic Chart of the Atoms. The mixtureis then sintered in vacuum with suflicient carbon to remove oxygen from the mixture by combining with it to form carbon dioxide. The sintering is continued until a porous compact of the homogeneous solid solution of the base and reactive metal is formed having a porosity between 15 and percent (i.e. a density between 70 and 85 percent of theoretical density). Next, the porous compact is heated in a nonoxidizing, nitrogen-containing atmosphere to a temperature which is high enough to permit uniform diffusion of the nitrogen intothe alloy.

The uniform distribution of nitrogen in the alloy is believed due to the permeation of the nitrogen gas into the pores of the compact and by solid state diffusion into the solid solution over relatively short distances. As a result of the diffusion, nitrides. of the reactive metals are precipitated -.in situ as fine particles (predominantice ly less than 0.1 micron in diameter) which are uniform- 1y.dispersed throughout the alloy.

After sintering, the alloy may be fabricated by any conventional metal Working process.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages. will become apparent from a study. of the following description in connection with the drawings, wherein:

titanium sheet containing nitrogen, and

FIG. 2 is a graph showing the relationship between the. percentage of nitrogen and the titanium content in a molybdenum-titanium alloy.

Our process for producing nitrogen containing alloys shall be described in detail for a molybdenum-titanium alloy, although it will beunderstood that it is also applicable to tungsten base alloys. Further, the reactive metal may be added as elemental titanium, hafnium or zirconium or as a hydride of titanium, hafnium, or zirconium.

' In preparingla molybdenum-titanium alloy, elemental powder of molybdenum and titanium or titanium hydride are mixed together, the amount of titanium in the alloy being between 0.25 and 3.0 percent. Sufficient carbon (approximately 500 to 1000 parts per million) is added to the mixture to remove any oxygen present by com bining with it to form carbon dioxide.

The-mixture is next sintered in a vacuum (less than 0.001 millimeter of mercury) and at a temperature in the range 1600-1900 C. for a period of between 180 and 20 minutes to form a porous homogeneous solid solution of molybdenum and titanium having a porosity in the range 15-30 percent. The carbon and oxygen remaining in the solid solution. should be less than parts per million.

The sintered solid solution is next heated in .a nonoxidizing nitrogen-containing atmosphere to'a tempera ture in the approximate range 13002.-000 C. until the desired amount of nitrogen is uniformly diffused into the alloy. As previously explained, the nitrogen distribution takes place by permeation of the gas into the pores and by solid state diffusion over short distances (on the order of 0.05 millimeter). The atmosphere may be pure nitrogen or, if desired, may consist of nitrogen plus a suflicient amount of hydrogen to prevent the formation of an oxide.

. The heating time required to obtain complete dispersion of the nitrogen through the alloy varies over a relatively Wide range as shown by the following table.

Nitriding above 1700 C. promotes the formation of coarse particles which are ineffective in strengthening.

After heating, the material is fabricated by conven- FIG. 1 is a photomicrograph of a molybdenumtonal methods, such as forging, rolling, drawing, or swaging. The temperatures used are somewhat higher than those normally employed in these operations due to the greater strength of the alloy. For example, forging is carried out in the range 1650-l700 C. and rolling in the range 13501500 C.

The resultant alloy has a higher recrystallization temperature and greater strength than unalloyed molybdenum and other comparable molybdenum base alloys. This is believed due to the creation of a uniformly distributed fine titanium nitride precipitate from the homogenized solid solution of molybdenum-titanium. FIG. 1, which is a photomicrograph (magnified approximately 95 times) of an as-rolled sheet of molybdenum-titanium alloy containing 0.5 percent titanium and 950 parts per million of nitrogen, shows the uniform fine grained structure obtained by this process.

It has been found that the production of a molybdenum-titanium-nitrogen alloy with a uniform dispersion of titanium nitride is not achieved with arc melting or other melting techniques. Also, the diffusion of nitrogen into a 100 percent dense section of a molybdenum-titanium solid solution (as described by Makherjee and Martin in an article Hardening of a Molybdenum Alloy by Nitride Dispersions, Journal of Less-Common Metals, 2 [1960], p. 392) produces a non-uniform dispersion at the surface of the alloy only.

FIG. 2 is a graph in which the specified amount of nitrogen in molybdenum base alloys having between 0.25 and 3 percent titanium is plotted against the percentage of titanium in the alloy. As shown, the minimum amount of nitrogen is 200 parts per million. The maximum amount of nitrogen is directly proportional to the percentage of titanium present in the alloy and equals the amount of nitrogen required to form stoichiometric titanium nitride with all of the available titanium.

The 100 percent recrystallization temperature after one hour was measured for molybdenum-0.5% titanium alloys having nitrogen contents varying from 28 parts per million to 1460 parts per million. The recrystallization temperature for alloys having less than 200 parts per million was less than 1400 C. whereas the 1 hour recrystallization temperature for alloys containing 200 parts per million or more exceeded 1400 C. in all cases and was above 1650 C. in some cases. Corresponding results were found in the titanium range 0.25 to 3 percent. By comparison, unalloyed molybdenum containing 25 parts per million of nitrogen had a recrystallization temperature below 1100 C.

The ultimate tensile strength at 1200 C. for molybdenum-titanium alloys of various compositions exceeded 40,000 p.s.i. for nitrogen contents between 200 parts per million and the amount required to transform all titanium to titanium nitride. By contrast, when the nitrogen content was reduced to 28 parts per million, the tensile strength at 1200 C. dropped to 26,000 p.s.i. For unalloyed molybdenum containing 25 parts per million nitrogen, the ultimate tensile strength was 11,000 p.s.i.

As many changes could be made in the above construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An alloy consisting essentially of a refractory metal selected from the group consisting of molybdenum and tungsten; a reactive metal chosen from the group con sisting of titanium, hafnium, and zirconium; and nitrogen, the amount of nitrogen in said alloy being between 200 parts per million and the amount required to transform all the contained reactive metal into stoichiometric nitride, said nitride being uniformly dispersed throughout the matrix as fine particles having a diameter predominantly less than 0.1 micron.

2. A molybdenum base alloy consisting essentially of molybdenum, nitrogen and between 0.25 and 3.0 percent titanium, the amount of nitrogen in said alloy being between 200 parts per million and the amount re- 5 quired to transform all the contained titanium into titanium nitride, said titanium nitride being uniformly dispersed throughout the matrix as fine particles having a diameter predominantly less than 0.1 micron.

3. The process of producing an alloy consisting essentially of nitrogen, a base material selected from the group consisting of molybdenum and tungsten, and a reactive metal selected from the group consisting of titanium, hafnium, and zirconium, said process comprising the steps of (a) mixing powders of said base material and said reactive element,

(b) sintering said mixture to form a solid solution in a compact, said compact having a porosity between 15 and 30 percent, and

(c) heating said solid solution in a non-oxidizing nitrogen-containing atmosphere until said nitrogen has been uniformly diffused through said solid solution, a nitride of said reactive metal being precipitated in situ as uniformly dispersed fine particles.

4. The process of producing an alloy consisting essentially of molybdenum, titanium, and nitrogen, said process comprising the steps of (a) mixing elemental powders of said molybdenum and said titanium,

(b) sintering said mixture to form a solid solution in a compact, said compact having a porosity between 15 and 30 percent, and

(c) heating said solid solution in a non-oxidizing nitrogen-containing atmosphere until said nitrogen has been uniformly diffused through said solid solution, titanium nitride being precipitated in situ as uniformly dispersed fine particles.

5. The process of producing an alloy consisting essentially of molybdenum, titanium, and nitrogen, said process comprising the steps of (a) mixing powders of molybdenum and titanium hydride,

(b) sintering said mixture to form a solid solution in a compact, said compact having a porosity between 15 and 30 percent, and

(c) heating said solid solution in a non-oxidizing nitrogen-containing atmosphere until said nitrogen has been uniformly diffused through said solid solu tion, titanium nitride being precipitated in situ as uniformly dispersed fine particles.

6. The process of producing an alloy consisting essentially of molybdenum, titanium, and nitrogen, said process comprising the steps of (a) mixing powders of said molybdenum and said titanium to form a mixture consisting of between 0.25 and 3.0 percent titanium,

(b) sintering said mixture at a temperature in the range 1600 to 1900" C. for between 180 and 20 minutes to form a solid solution in a compact, said 60 compact having a porosity between 15 and 30 percent, and

(c) heating said solid solution ina non-oxidizing nitrogen-containing atmosphere to a temperature in the range 1500 to 2000" C. until said nitrogen has been uniformly diffused through said solid solution, titanium nitride being precipitated in situ as uniformly dispersed fine particles.

7. The process of producing an alloy consisting essentially of molybdenum, titanium, and nitrogen, said process comprising the steps of (a) mixing powders of said molybdenum and said titanium to form a mixture consisting of between 0.25 and 3.0 percent titanium,

(b) adding between 500 and 1000 parts per million trogen-containing atmosphere to a temperature in the range 1500-2000 C. until said nitrogen has been uniformly diffused through said solid solution.

References Cited in the file of this patent UNITED STATES PATENTS Laise June 7, 1927 Walter June 28, 1932 Funkhouser Mar. 6, 1962 

1. AN ALLOY CONSISTING ESSENTIALLY OF A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN; A REACTIVE METAL CHOSEN FROM THE GROUP CONSISTING OF TITANIUM, HAFNIUM, AND ZIRCONIUM; AND NITROGEN, THE AMOUNT OF NITROGEN IN SAID ALLOY BEINB BETWEEN 200 PARTS PER MILLION AND THE AMOUNT REQUIRED TO TRANSFORM ALL THE CONTAINED REACTIVE METAL INTO STOICHIOMETRIC NITRIDE, SAID NITRIDE BEING UNIFORMLY DISPERSED THROUGHOUT THE MATRIX AS FINE PARTICLES HAVING A DIAMETER PREDEOMINANTLY LESS THAN 0.1 MICRON. 